void main() {
  List<int> arr = [1, 2, 3];
  print(arr); //[1,2,3]
  print(arr.length); //3
  for (var value in arr) {
    print(value); //依次打印 1,2,3
  }

  arr.forEach((ele) => {print(ele)}); //1,2,3,只有1个参数
  print(arr[0]); //1
  arr.add(4);
  arr.add(5);
  print(arr); //[1, 2, 3, 4, 5]
}
